Au contraire friend........ an IG who does his job will have built a crushing case before the appointee or senior boss who is no longer there is even interviewed.
Bosses hold meetings with underlings and tell them to do things on behalf of the boss or at their direction. The underlings are career employees and they want their retirement. Bosses send emails and text messages on their work phones. The IG has unfettered access to all of this. Solid investigations involving groups of people are built from the bottom up - foot soldier says Lt told him to do XYZ and said ABC. Lt says Captain told him to do XYZ and said ABC. Captain says Colonel said to do XYZ and said ABC. Colonel who is in meeting with appointed General says the same.
The IG also has unfettered access to internal emails, calendars, meeting notes, admin information, etc. The career employees generally have a pretty good idea of what the bosses are doing and they are only exposed to repercussions if they lie.
Based on this, the IG is a pretty powerful tool in this game. By the time a criminal investigator interviews “the target” they will have dozens of interviews and documents from the IG about what happened in their back pocket. The government runs on paperwork and unlike congress - the IG gets it all.
crooks committing crimes keep their communications limited as much as possible...less possible witnesses...come on man!